RFID tag using a surface insensitive antenna structure

ABSTRACT

An RFID device includes conductive tabs, and a conductive structure, with a dielectric layer between the conductive tabs and the conductive structure. The conductive structure overlaps the conductive tabs and acts as a shield, allowing the device to be at least somewhat insensitive to the surface upon which it is mounted, or to the presence of nearby objects, such as goods in a carton or other container that includes the device. The dielectric layer may be a portion of the container, such as an overlapped portion of the container. Alternatively, the dielectric layer may be a separate layer, which may vary in thickness, allowing one of the conductive tabs to be capacitively coupled to the conductive structure. As another alternative, the dielectric layer may be an expandable substrate that may be expanded after fabrication operations, such as printing.

This is a continuation of International Application No. PCT/US04/11218,filed Apr. 12, 2004, published in English as WO 2004/093246, whichclaims priority from U.S. Provisional Application No. 60/517,156, filedNov. 4, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of Radio Frequency Identification(RFID) tags and labels, and in particular to tags and labels thatinclude a surface insensitive antenna structure.

2. Description of the Related Art

Current inventory and manufacturing methods rely on the ability to trackand identify items of concern, such as inventory items, packages,individual parts, etc., and to communicate information concerning suchitems of concern in a wireless manner for processing and use. One knownmethod of tracking and providing information on such items of concern isto attach to each such item a wireless communication device, such as anactive or passive transponder, incorporated into an identification tagor label that responds to radio frequency interrogation and commands.The tag may store or represent information concerning the item to whichit is attached, such as a unique identifying number, item status such asopened or unopened, location, and the like. The tag may be attached toan individual item or to packaging containing multiple items.

The present invention deals with the problems that arise in attemptingto design and manufacture an RFID tag that has general applicability andcan efficiently operate when the packaging and containers on which sucha tag may be attached vary widely. For example, some items of interestare shipped in bulk in a single container made of cardboard or plastic,heavier items may be shipped in wooden boxes, and liquids and viscousmaterials may be shipped in metal containers. Specifically, the presentinvention is directed toward meeting the problems that arise inattempting to design and manufacture an antenna structure that willoperate efficiently and properly over a wide range of such packaging andcontainers.

In general, an antenna connected to an RFID tag is designed foroperation on a specific or narrow range of substrates on which it may beattached or upon which it may be otherwise coupled to. Other substrateswill cause the radiation efficiency of the antenna to deteriorate fromthe designed optimal mounting substrates. Thus, the antenna, andconsequently the tag, will no longer radiate properly as designed. Thisloss of antenna efficiency may be due to a number of variable packagingfactors. One is that each substrate has its own dielectriccharacteristics that typically affect the impedance matching between thewireless communication device and its antenna. Impedance matchingensures the most efficient energy transfer between an antenna and thewireless communication device.

The substrates on which such antennas are attached therefore areimportant in designing the antenna and the subsequent operatingefficiency of the RFID tag itself. Common substrates vary from thenon-conductive, such as cardboard, to the conductive, such as foil. Infact, even within a single type of substrate the dielectriccharacteristics may vary, such as in cardboard packaging, the thicknessof the cardboard substrate can differ from package to package, even themoisture content of the cardboard due to weather changes may cause achange in its dielectric characteristics. The radiation efficiency andoperation of the antenna can be affected by these highly variablefactors even if the antenna was designed for attachment to a cardboardsubstrate. Therefore, a need exists to provide an antenna for use with awireless communication device in an RFID tag whose impedance andtherefore radiation efficiency is substantially insensitive to thesubstrate on which it may be attached.

Some prior art systems attempt to overcome the problem of the varyingdielectric characteristics of the packaging on which the tag is appliedby mounting and encapsulating the wireless communication device andantenna system on a known substrate and then attaching the encapsulatedsystem onto the subject packaging so that the RFID tag “floats”independently of the packaging on which it is mounted, i.e., it isseparated from the package by some distance, such as label/ticket thatis attached with a plastic fastener. The problem with this encapsulatedRFID tag is that it is cumbersome, expensive, difficult to mount andprotrudes from the surface of the packaging on which it is mounted,leaving it susceptible to damage and prone to fall off during handlingof the packaging, leaving the item untagged.

To overcome these noted problems, RFID tag embodiments have beendeveloped that are directly attached by adhesion to or printing on thesurface of the packaging thereby resulting in a lower tag profile thatis less prone to damage or removal during handling of the packaging.However, direct surface mounting of these antennas and tags cause theirefficiency to suffer from the varying dielectric characteristics of thesurfaces on which they are attached. Thus, these systems require thatdifferent tags be used on different packaging resulting in added cost,complexity and manufacturing inconvenience. Since it is preferable totag each inventory item separately, the need for so many different tagsmultiples the problems greatly.

Other known prior art RFID tags have attempted to overcome the problemof the varying dielectric characteristics of the packaging and provide agenerally, “one size fits all” tag, by designing tag antenna systemsthat are insensitive to the surfaces on which they are mounted.

One known such RFID tag structure that is insensitive to its mountingsurface, is that described in U.S. Pat. No. 6,501,435 to King et al.,titled “Wireless Communication Device and Method.” This tag structurecompensates for the varying substrate dielectric characteristics onwhich the RFID tabs are attached by utilizing an antenna structure wherethe radiating tabs are asymmetric with regard to their shape and size.The tab(s) may be attached to the surface of the package or dielectricmaterial opposite the tab(s).

SUMMARY OF THE INVENTION

One aspect of the present invention relates to an antenna system and anRFID tag or label that is insensitive to or compensates for thesubstrate on which it is mounted.

The RFID tag includes a wireless communication device, such as a passiveor active transponder that responds to a radio frequency signal tocommunicate stored information concerning a package or its contents.

The antenna system contains two or more tabs constructed out ofconductive material. The tabs may act as a monopole or multiple arrayantenna and may further act to attach the wireless communication deviceof the tag to the packaging or container. Likewise the tabs may resonateat one or multiple frequencies of interest. Apertures in the packagingmay be used in conjunction with the tabs to form slot antennas orantennas that have multiple resonant frequencies.

The tabs may be attached to, be printed on, or be formed by the surfaceof the package. In one embodiment, the tab(s) are attached to thesurface of a package, which itself comprises a dielectric material. Anexample of this embodiment is attachment of the tabs to a corrugatedcardboard box. This embodiment offers the advantage of efficiency, byeliminating the need for an additional dielectric material beyond thatof the package itself. A disadvantage of this is that variability of thepackaging material, such as its structure and its dielectric constant(the dielectric constant is the amount of permissivity of a particularmaterial). This variability in turn can lead to unacceptable variationsin the impedance of the RFID device.

Therefore, the tab(s) may be reactively coupled to the surface of thepackage through a dielectric material. The dielectric material can be anadhesive coated flexible label stock on which the tabs are mounted toattach the RFID device to a package. In one embodiment of the invention,the label stock is a flexible material, such as a polymeric film, thatis manufactured in long rolls of flexible web material usingroll-to-roll manufacturing techniques. The uniform tab design of thisembodiment, as further explained herein, offers various manufacturingadvantages in roll-to-roll manufacturing of label stock, such assimplifying the printing or other formation of the tabs; simplifyingconnection of the tabs to the microelectronic device; facilitating diecutting or other separation into individual labels or label inlays; andother advantages as are apparent to those skilled in the art.

The dielectric material of the package, or alternatively the dielectricmaterial of the RFID tag, includes a conductive structure such as aground plane opposite the conductive tab(s), which may act as a radiofrequency reflecting shield.

Methods for manufacturing RFID labels are disclosed in PCT PublicationNo. WO 01/61646 by Moore North America, Inc., incorporated herein bythis reference. The method disclosed in PCT Publication No. WO 01/61646uses a number of different sources of RFID inlets, each inlet includingan antenna and a chip. A plurality of webs is matched together and RFIDlabels are die cut from the webs, to produce RFID labels with a liner.Alternatively, linerless RFID labels are produced from a composite webwith a release material on one face and pressure sensitive adhesive onthe other, the labels formed by perforations in the web. Variousalternatives are possible.

The tabs may be formed on the web using a wide variety of materials andprocesses. For example, one process involves printing on the web aconductive material, such as silver conductive ink, in a patterndefining multiple tabs. The ink may be printed for example using silkscreening techniques, such as in a sheet fed or roll operation. The tabsare typically dried and stored on the web in a roll. However, as analternative, the tabs may be wet printed during the label manufacturingprocess, in line with other process steps.

Additional suitable methods of forming the tabs include printingconductive ink, sputtering metal, laminating foil or hot-stamping, orany method known in the art for forming conductive patterns on a film.

The precision or definition of the printed elements of lines and spacesmay be important to the performance of the tabs and the overall RFIDdevice. With some tab designs, conventional printing may not provideadequate resolution, line/space separation or other qualitycharacteristics necessary to deliver engineered performance.

Likewise, control of thickness and smoothness of the printed areas ofthe tabs can have an important effect on their performance. Variabilitydue to ink formulation, environmental conditions, substratespecifications, process conditions and other factors can impact both thesmoothness and final thickness of printed antennas. Surface tensioneffects underlie many of these variables and place constraints on theamount of ink that can be deposited, and how closely graphic elementscan be positioned to one another.

In addition to a flexible dielectric base material that carried thetabs, an additional continuous web or sheet of selected materials may beprovided to support and protect the tabs and microelectronic device,and/or to provide usable form factors and surface properties (e.g.printability, adhesive anchorage, weatherability, etc.) for specificapplications. The base material and additional protective material(s)may be made of films, papers, laminations of films and papers, or otherflexible sheet materials suitable for a particular end use. Theresulting continuous web of RFID label stock or RFID tag stock may beoverprinted with text and/or graphics, die-cut into specific shapes andsizes into rolls of continuous labels, or sheets of single or multiplelabels, or rolls or sheets of tags.

In typical label constructions, the label is die cut, as with a wedgedie or other cutting method known in the label art. In the case of apressure sensitive adhesive label carried on a liner layer, the die cutmay extend all the way through the cross-section of the label or the cutmay extend only down to the liner layer. In this instance, the liner maybe kept as a unified sheet of standard sheet size, with one or moreremovable labels on top of the sheet, as is typical in the labeling art.It is noted that an adhesive layer and corresponding release liner maybe omitted, in the event that a tag rather than a label is desired.

In one embodiment, the label stock is a foam material as a foam materialprovides electrical as well as mechanical manufacturing advantages inproviding a more efficient performance than some other materials such ascardboard, within relatively short, compact dimensions than wouldotherwise be required to produce the same results. The foam materialalso acts as a dielectric medium, as discussed further below, that makesfor easier manufacturing and permits laminating flexibility and lesscost than found with solid materials such as polypropylene which arefairly inflexible and costly in manufacturing and use.

Also, the manufacturing advantages of uniform tabs are seen to greatestadvantage when the labels or tab sets as they are being manufactured ona web medium, are arranged on such medium in a regular array, such as anorthogonal row-and-column array. Just as the geometry of one embodimentinvolves one or more rows of tabs arrayed along the machine direction ofa web of flexible dielectric stock, according to one embodiment each setof uniform tabs may be circumscribed by a well-defined area of the web.Most preferably this circumscribing area substantially takes the form ofa rectangle.

Likewise, a ground plane or radio frequency reflecting member of theantenna structure or tag may be formed in association with the tagitself for better space and manufacturing efficiency using the webmanufacturing process. In one method, the ground plane is formed orprovided separately from the formation of the tabs on the long flexibleweb. In this case, the separately produced ground plane must later beassociated with and properly positioned when the tag is placed on itsdesired packaging.

In another method, the ground plane is formed on the same web as thetabs, preferably along the machine direction of the web. The antennatabs and the ground plane may be separated by a fold line, which may bea scored line on the web, for ease in application of the label topackaging. In this method, the antenna structure of the tag and itsassociated ground plane elements are formed together in a known andfixed spatial relationship and handled as a single unit for applicationto the desired packaging. In application, the tag is placed on a side ofthe packaging along an edge. In this position, the tag can be foldedalong a score line so that the antenna structure portion of the tagremains on the outside surface panel of the package, while the groundplane portion is positioned on the inside surface of the panel of thepackage opposite the antenna tabs to provide a radio frequencyreflective ground plane. Intermediate the antenna tabs and the groundplane element of the tag is sandwiched the side of the packaging, whichmay be used by the label as a dielectric between these two labelelements for better label operation.

In another embodiment, the tabs may be printed on a continuous webbingwith a self-adhesive backing. When used, they are detached from thewebbing and attached to the packaging in an automated procedure. In adifferent embodiment, the tabs are reactively coupled to the surface ofthe package through a dielectric material. The dielectric material mayinclude an adhesive material placed on the tabs that doubles to attachthe tag to a package. Alternatively, the dielectric material may includethe material from which the package is constructed, such as a cardboardlayer between the tabs and a grounding or radio frequency reflectingstructure commonly referred to as a ground plane.

The antenna structure may be configured so that the impedance of theantenna system is not substantially affected by the substrate to whichthe wireless communication device is attached, so that it will remainwithin a known range of impedance for tag designing purposes. In oneembodiment, the antenna arrangement is a dipole antenna formed byidentically shaped tabs. The tabs are manufactured with an adhesive onone side so that they may be adhered to the surface of the substrateforming the packaging. The tabs are connected at feedpoints to thewireless communication device with transmission lines that may beconductive paths or wires.

It is also contemplated that the wireless communication device of theRFID tag may be recessed into an indentation formed in the packagingsubstrate so that the wireless communication device does not protrudefrom the substrate surface, making the RFID tag less prone to damageduring handling of the packaging.

In another embodiment, the invention includes a tag that may be mountedon only one side of the packaging. In this embodiment at least twoconductive tabs are arranged to form a dipole antenna. A thin dielectricis coupled to the conductive tabs and a ground plane, or radio frequencyreflecting structure, is coupled to the thin dielectric so that the thindielectric is between the conductive tabs and ground plane. The groundreflecting structure can be unitary, that is, formed from a singleconnected element such as a flat plate, or formed from a cooperatingseries of isolated components such as a series of non-connected flatplates.

According to an aspect of the invention, a radio frequencyidentification (RFID) device includes a pair of conductive tabs on afirst surface of the dielectric layer; a conductive structure; and adielectric layer between the conductive tabs and the conductivestructure. The dielectric layer has a thinner portion and a thickerportion. One of the conductive tabs is at least partially on the thinnerportion of the dielectric layer.

According to another aspect of the invention, a method of making an RFIDdevice includes: forming a pair of conductive tabs on one face of adielectric layer; and forming a thinner portion of the dielectric layerthat is thinner than a thicker portion of the dielectric layer. One ofthe conductive tabs is at least partially on the thinner portion.

According to yet another aspect of the invention, an RFID deviceincludes a pair of dielectric layers joined together to create anoverlapping portion; a pair of conductive tabs on one of the dielectriclayers, at the overlapping portion; and a conductive structure on theother of the dielectric layers, at the overlapping portion, thedielectric layers of the overlapping portion thereby being between theconductive structure and the conductive tabs.

According to still another aspect of the invention, a method of applyingan RFID device to a container includes forming a overlapping portion ofthe container by overlapping and joining together a pair of parts of thecontainer; forming a pair of conductive tabs at the overlapping portionon one of the parts of the container; and forming a conductive structureat the overlapping portion at the other of the parts of the container.

According to a further aspect of the invention an RFID device includes:an expandable substrate; and an antenna structure on one face of thesubstrate.

According to a still further aspect of the invention, an expandablesubstrate includes a middle film having multiple segments. A first partof each segment is attached to a top film and a second part of eachsegment is attached to a bottom film. A third part of each segment,between the first and second parts, rotates relative to the first andsecond parts when the top film is moved relative to the bottom film,thereby expanding the substrate.

According to another aspect of the invention, a method of forming anRFID device includes: printing one or more layers of the device atop anexpandable substrate; and expanding the substrate, thereby increasingthe thickness of the substrate.

To the accomplishment of the foregoing and related ends, the inventioncomprises the features hereinafter fully described and particularlypointed out in the claims. The following description and the annexeddrawings set forth in detail certain illustrative embodiments of theinvention. These embodiments are indicative, however, of but a few ofthe various ways in which the principles of the invention may beemployed. Other objects, advantages and novel features of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings, which are not necessarily according to scale,

FIG. 1 is a schematic diagram of an RFID tag incorporating an antennaarrangement embodying the present invention;

FIG. 2 is a schematic diagram of an RFID tag incorporating analternative antenna arrangement embodying the present invention;

FIG. 3 is a schematic diagram of an RFID tag incorporating a secondalternative antenna arrangement embodying the present invention;

FIG. 4 is a cross sectional view of an RFID tag incorporating an antennaarrangement embodying the present invention as it would be mounted on apackaging sidewall;

FIG. 5 is a cross sectional view of an RFID device of the presentinvention mounted on an overlapping portion of a carton;

FIG. 6 is an oblique view of a marker printed on a portion of a cartonor other container, indicating where a reflective conductive structureis to be located;

FIG. 7 is an oblique view illustrating placement of the RFID device ofFIG. 6;

FIG. 8 is an oblique view of an RFID device in accordance with thepresent invention, having a monopole antenna structure;

FIG. 9 is a plan view of one embodiment of the RFID device of FIG. 8;

FIG. 10 is an oblique view of another embodiment of the RFID device ofFIG. 8;

FIG. 11 is a schematic view showing a system for producing the RFIDdevice of FIG. 8;

FIG. 12 is a cross sectional view of an RFID device in accordance withthe present invention, having an expandable substrate;

FIG. 13 is an exploded view of the expandable substrate of the device ofFIG. 12;

FIG. 14 is an oblique view of the expandable substrate of the device ofFIG. 12, in a compressed state;

FIG. 15 is an oblique view of the expandable substrate of the device ofFIG. 12, illustrating expansion of the substrate; and

FIG. 16 is a plan view of an RFID device in accordance with the presentinvention, having generally rectangular conductive tabs.

DETAILED DESCRIPTION

The present invention is directed to a radio frequency identificationdevice (RFID) and its antenna system as it is attached to a package orcontainer to communicate information about the package or container toan external reader. The package may be an individual package containingspecific, known contents, or an individual, exterior package containingwithin it a group of additional, interior individual packages. The word“package” and “container” are used interchangeably herein to describe amaterial that houses contents, such as goods or other individualpackages, and equivalent structures. The present invention should not belimited to any particular meaning or method when either “package” or“container” is used.

An RFID device includes conductive tabs, and a conductive structure,with a dielectric layer between the conductive tabs and the conductivestructure. The conductive structure overlaps the conductive tabs andacts as a shield, allowing the device to be at least somewhatinsensitive to the surface upon which it is mounted, or to the presenceof nearby objects, such as goods in a carton or other container thatincludes the device. The dielectric layer may be a portion of thecontainer, such as an overlapped portion of the container.Alternatively, the dielectric layer may be a separate layer, which mayvary in thickness, allowing one of the conductive tabs to becapacitively coupled to the conductive structure. As anotheralternative, the dielectric layer may be an expandable substrate thatmay be expanded after fabrication operations, such as printing.

FIG. 1 illustrates one embodiment of the present invention that is foundin an RFID tag 10 that includes a wireless communication device 16. Thedevice 16 may be either active in generating itself the radio frequencyenergy in response to a received command, or passive in merelyreflecting received radio frequency energy back to an externaloriginating source, such as current RFID tag readers known in the art.

In this embodiment, there are preferably at least two conductive tabs 12and 14, coupled to the wireless communication device for receiving andradiating radio frequency energy received. The tabs 12 and 14 togetherform an antenna structure 17. The two tabs 12 and 14 are substantiallyidentical in shape and are coupled to the wireless communication device16 at respective feedpoints 20 and 22 that differ in location relativeto each of the tabs. The tabs 12 and 14 may be generally identical inconducting area if the two tabs are of the same size as well as shape.Alternatively the tabs 12 and 14 may differ in size while their shaperemains generally the same resulting in a different conducting area. Thetabs 12 and 14 may be collinear or non-collinear to provide differentdesired antenna structures. For example, in FIG. 1 tabs 12 and 14 areoffset and adjacent to provide a slot antenna system in area 18 thatprovides for resonance at multiple radiating frequencies for operationat multiple frequencies.

It is also contemplated that the invention includes having multiplearrays of conductive tabs that are connected to device 16. These tabsmay be designed to work in unison with one another to form dipole orYagi antenna systems, or singly to form monopole antennas as desired forthe particular tag application. By using such multiple conductive tabarrays, multiple resonant frequencies may be provided so that the tagmay be responsive to a wider range of tag readers and environmentalsituations than a single dedicated pair of conductive tabs. It will beappreciated that multiple conductive tabs to improve the range ordirectional characteristics in reading the RFID device.

Other considered shapes for the conductive tabs are illustrated in FIGS.2 and 3 and include not only regular shapes such as the tapered,triangular shape illustrated in FIG. 1, but also truncated triangularshapes denoted by reference numbers 32 and 34 in FIG. 3.

Rectangular shaped conductive tabs are also included in this inventionas illustrated in FIG. 2 as reference numbers 22 and 24. In fact, FIG. 2illustrates, for example, that the tabs may include a series ofcontiguous rectangular portions 26, 27, 28 and 40, 41, 42.

In one embodiment of the invention, the rectangular portions shown inFIG. 2 will have dimensions substantially as follows: Rectangularportion 26 is about 3 millimeters wide by about 3 millimeters long;contiguous rectangular portion 27 is about 10 millimeters wide by about107.6 millimeters long; and, rectangular portion 28 is about 3millimeters wide by 15.4 millimeters long. With these dimensions, it isfurther preferred that the dielectric substrate have a thickness betweenthe conductive tabs and the ground plane of about 6.2 millimeters forfoam. Likewise, the ground plane for this preferred embodiment is about16 millimeters wide by about 261 millimeters long.

The conductive tabs may also have irregular shapes, or even compositeshapes that include both regular and irregular portions. Otheralternative antenna systems that embody the present invention includethose that have tabs with a triangular portion contiguous with afreeform curve or a regular curve such as a sinusoidal pattern.

In FIG. 1, the tab feedpoints 20 and 22, may be selected so that theimpedance across the two feedpoints 20 and 22 of tabs 12 and 14,respectively, is a conjugate match of the impedance across the wirelesscommunication device 16 to allow for a maximum transfer of energytherebetween.

In general, a method of selecting feedpoints on the tabs to achieve thisconjugate impedance match, may be to select points on each tab differingin location where the width profile of each tab, taken along an axistransverse to the longitudinal centerline axis of each tab, differs fromone another. That is, the feedpoints 20 and 22 may be selected such thatthe width of the tabs 12 and 14 at the feedpoints 20 and 22, taken alongthe centerline of the tab as you move away from the center of the tagwhere it connects to the communications device, measured against thelength, differs between the two tabs 12 and 14. By choosing such points,either by calculation or trial and error, a conjugate impedance matchcan be achieved.

Specifically, with reference to the Figures, the longitudinal centerlineaxis of a tab is seen to be a line that remains equidistant fromopposite borders or edges of the tab and extending from one end of thetab to the other. At times with regular shaped tabs, this longitudinalcenterline axis will be a straight line similar to a longitudinal axisof the tab. At other times, with irregular shaped tabs, the longitudinalcenterline axis will curve to remain equidistant from the borders. It isalso seen that this longitudinal centerline axis is unique to each tab.The width of the tab is determined along an axis transverse to thelongitudinal centerline axis and will be seen to be dependent upon theshape of the tab. For example, with a rectangular shaped tab, the widthwill not vary along the longitudinal centerline axis, but with atriangular or wedge shaped tab, the width will vary continuously alongthe longitudinal centerline axis of the tab. Thus, while it iscontemplated that the present invention includes tabs having rectangularshaped portions, there will also be portions having different widths.

Another method of selecting the feedpoints on the conductive tabs, is toselect a feedpoint differing in location on each of the tabs where theconducting area per unit length of the longitudinal centerline axis ofeach tab varies with distance along the longitudinal centerline axis ofeach of said tabs from its feedpoint. In essence, this method selects asa feedpoint a location on each tab where the integrated area of theshape per unit length of the centerline varies and is not necessarilythe width of the tab.

FIG. 4 illustrates how a radio frequency reflecting structure or groundplane 50 is coupled to tabs 52 and 54, for reflecting radio frequencyenergy radiated from the tabs 52 and 54. The ground plane elements maybe substantially the same size as the conductive tabs or greater, sothat the ground plane elements may effectively reflect radio frequencyenergy. If the ground plane elements are substantially smaller than theconductive tabs, the radio frequency energy will extend beyond the edgesof the ground plane elements and interact with the contents of thepackaging causing deterioration in the operating efficiency of thelabel. In one embodiment, the ground plane 50 may extend at least about6 mm beyond the boundary of the tabs 52 and 54.

In the illustrated embodiment the wireless communication device 56 isconnected at feedpoints 58 and 60 to the tabs 52 and 54. This structure50 may be a simple ground plane made from a single, unitary plate or acomplex reflecting structure that includes several isolated plates thatact together to reflect radio frequency energy. If the antenna structureis located on one side of a package wall 62, the radio frequencyreflecting structure 50 may be on the opposite side of the same wall 62using the wall itself as a dielectric material as described furtherbelow.

As indicated above, a dielectric material is preferably locatedintermediate the conductive tabs 52 and 54, and the radio frequencyreflecting structure 50. An example of such a dielectric material is thepackaging wall 62 described above. The thickness or the dielectriccharacteristic of the dielectric intermediate the tabs and radiofrequency reflecting structure may be varied along a longitudinal ortransverse axis of the tabs. Generally, it has been found that at UHFfrequencies, defined as a band in the range of 860 MHz to 950 MHz, adielectric thickness of about 3 millimeters to 6 millimeters is suitablefor a tag embodying the present invention. Likewise, a dielectricthickness of about 0.5 millimeter to about 3 millimeters is suitable fora tag designed to operate in a band centered on 2450 MHz. This range ofthickness has been found to be suitable for efficient operation of theconductive tabs despite the normally believed requirement for aseparation distance of a quarter of a wavelength of the operatingfrequency between the radiating element and ground plane.

With the present invention advantages have been found in bothmanufacturing and application of the labels in that a thinner, lowerdielectric material may be used in label construction, as well as thefact that shorter tabs may be utilized resulting in a manufacturingsavings in using less ink and label materials in constructing each labeland in increasing the label density on the web medium duringmanufacturing making less wasted web medium. Also such thinner andsmaller labels are easier to affix to packaging and less likely to bedamaged than those thicker labels that protrude outwardly from thepackaging surface to which they are attached.

Another embodiment of the present invention is directed toward theantenna structure itself as described above without the wirelesscommunication device.

Turning now to FIG. 5, an RFID device 70 is illustrated mounted on parts72 and 74 of a carton 76. The device 70 is located on an overlappingportion 80 of the carton 76, where the parts 72 and 74 overlap oneanother. The parts 72 and 74 may be adhesively joined in the overlappingportion. Alternatively, the parts 72 and 74 of the carton 76 may bejoined by other means, such as suitable staples or other fasteners. Onone side or major face 78 of the overlapping portion 80 are conductivetabs 82 and 84, and a wireless communication device 86, such as an RFIDchip or strap. A radio frequency reflecting structure or ground plane 90is on an opposite side or major face 92 of the overlapping portion 80.

The overlapping portion 80 of the carton 76 thus functions as adielectric between the conductive tabs 82 and 84, and the wirelesscommunication device 86. Performance of the RFID device 70 may beenhanced by the additional thickness of the overlapping portion 80,relative to single-thickness (non-overlapped) parts of the carton parts72 and 74. More particularly, utilizing a double-thickness overlappedcarton portion as the dielectric for an RFID device may allow for use ofsuch devices on cardboard cartons having thinner material. For example,some cartons utilize a very thin cardboard, such as 2 mm thickcardboard. A single thickness of 2 mm thick cardboard may be unsuitableor less suitable for use with surface-insensitive RFID device such asdescribed herein.

The RFID device 70 shown in FIG. 5 may be produced by printingconductive ink on the opposite sides (major faces) 78 and 92 of theoverlapping portion 80, to form the conductive tabs 82 and 84, and thereflecting structure 90. It will be appreciated that a variety ofsuitable printing methods may be used to form the tabs 82 and 84, andthe reflecting structure 90, including ink jet printing, offsetprinting, and Gravure printing.

The wireless communication device 86 may be suitably joined to theconductive tabs 82 and 84 following printing of the conductive tabs 82and 84. The joining may be accomplished by a suitable roll process, forexample, by placing the communication device 86 from a web of devicesonto the tabs 82 and 84.

It will appreciated that the printing may be performed before the cartonparts 72 and 74 are overlapped to form the overlapping portion 80, oralternatively that the printing may in whole or in part be performedafter formation of the overlapping portion 80. The conductive ink may beany of a variety of suitable inks, including inks containing metalparticles, such as silver particles.

It will be appreciated that formation of the conductive tabs 82 and 84,and/or the reflective structure 90 may occur during formation of thecarton parts 72 and 74, with the conductive tabs 82 and 84 and/or thereflective structure 90 being for example within the carton parts 72 and74. Forming parts of the RFID device 70 at least partially within thecarton parts 72 and 74 aids in physically protecting components of theRFID device 70 from damage. In addition, burying some components of theRFID device 70 aids in preventing removal or disabling of the RFIDdevice 70, since the RFID device 70 may thereby be more difficult tolocate.

In one embodiment, the conductive tabs 82 and 84 may be printed onto theinterior of the carton parts 72. As illustrated in FIG. 6, a marker 96may be printed or otherwise placed on one of the carton parts 72 and 74to indicate where the reflective structure 90 is subsequently to beplaced.

The conductive tabs 82 and 84 may have any of the suitable shapes orforms described herein. Alternatively, the conductive tabs 82 and 84 mayhave other forms, such as shapes that are asymmetric with one another.The conductive tabs 82 and 84 may have configurations that are tunableor otherwise compensate for different substrate materials and/orthicknesses, and/or for other differences in the environment encounteredby the RFID device 70, such as differences in the types of contents in acarton or other container on which the RFID device 70 is mounted.

The RFID devices 70 illustrated in FIGS. 5 and 6 enable mounting ofdevices on a wider range of packaging materials, with the reflectivestructure 90 providing a “shield” to reduce or prevent changes inoperation of the RFID device 70 due to differences in the types ofmerchandise or other material stored in a carton or other container uponwhich the RFID device 70 is mounted. As illustrated in FIG. 7, the RFIDdevice 70 may be located on a carton or other container 98, oriented sothat the reflective structure 90 is interposed between the conductivetabs 82 and 84, and the interior of the container 98.

FIG. 8 shows the operative components of another embodiment RFID device,an RFID device 100 having an essentially monopole antenna structure 102.The RFID device 100 includes a wireless communication device 106 (e.g.,a strap) that is coupled to a pair of conductive tabs 108 and 110 thatare mounted on a substrate 112, with a reflective structure or groundplane 114 on an opposite side of the substrate 112 from the conductivetabs 108 and 110.

At least part of one of the conductive tab 108 is capacitively coupledto the reflective structure 114, by being mounted on a thinner portion116 of the substrate 112, which has a thickness less than that of theportion of the substrate 112 underlying the conductive tab 110. It willbe appreciated that, with proper attention to matching, electricallycoupling the tab 108 to the conductive reflective structure 114, allowsoperation of the RFID device 100 as a monopole antenna device. Therelative thinness of the thinner portion 116 facilitates capacitiveelectrical coupling between the conductive tab 108 and the conductivereflective structure 114.

The conductive tab 110 functions as a monopole antenna element. Theconductive tab 110 may have a varying width, such as that describedabove with regard to other embodiments.

The matching referred to above may include making the relativeimpedances of the antenna structure 102 and the wireless communicationdevice 106 complex conjugates of one another. In general, the impedanceof the antenna structure 102 will be a series combination of variousimpedances of the RFID device 100, including the impedance of theconductive tab 108 and its capacitive coupling with the reflectivestructure 114.

The thinner portion 116 may be made thinner by inelastically compressingthe material of the substrate 112. For example the substrate 112 may bemade of a suitable foam material, such as a suitable thermoplastic foammaterial, which may be a foam material including polypropylene and/orpolystyrene. A portion of the substrate 112 may be compressed byapplying sufficient pressure to rupture cells, causing the gas in thecells to be pressed out of the foam, thereby permanently compressing thefoam.

The compressing described above may be performed after the formation ofthe tabs 108 and 110 on the substrate 112. The pressure on the tab 108and the portion of the substrate 112 may be directed downward andsideways, toward the center of the RIFD device 100, for example wherethe wireless communication device 106 is mounted. By pressing down andin on the conductive tab 108 and the substrate 112, less stretching ofthe material of the conductive tab 108 occurs. This puts less stress onthe material of the conductive tab 108, and may aid in maintainingintegrity of the material of the conductive tab 108.

As an alternative, it will be appreciated that the conductive tabs 108and 110 may be formed after compression or other thinning processes toproduce the thinned portion 116 of the substrate 112. The conductivetabs 108 and 110 may be formed by suitable processes for depositingconductive material, such as by printing conductive ink.

With reference again to FIG. 8, the substrate 112 may have a slopedregion 120 between its thicker portion 122 and the thinner portion 116.The sloped region 120 may aid in reducing stresses on the conductive tab108 when the conductive tab 108 is placed prior to compressing of thethinner portion 116, by increasing the area of the conductive tab 108that is under stress. When the thinner portion 116 is compressed priorto printing or other depositing of the conductive tab 108, the slopedregion 120 may aid in ensuring conduction between a first part 132 ofthe conductive tab 108 that is on the thicker portion 122 of thesubstrate 112, and a second part 136 of the conductive tab 108 that ison the thinner portion 116 of the substrate 112.

It will be appreciated that a variety of suitable methods may beutilized to produce the thinner portion 116 of the substrate 112. Inaddition to the compressing already mentioned above, it may be possibleto heat a portion of the substrate, either in combination withcompression or alone, to produce the thinner portion 116. For example, athermoplastic foam material may be heated and compressed by running itthrough a pair of rollers, at least one of which is heated. Thethermoplastic film may be compressed over an area, and turned into asolid thermoplastic sheet, thus both reducing its thickness andincreasing its dielectric constant. Alternatively, material may beremoved from a portion of the substrate 112, by any of a variety ofsuitable methods, to produce the thinner portion 116.

As suggested above, the proximity of the second conductive tab part 136to the conducting reflective structure 114, with only the thinnerportion 116 of the substrate 112 between, aids in capacitively couplingthe second part 136 and the reflective structure 114. In a specificexample, a 3.2 mm thick foam dielectric was compressed over a 20 mm×10mm area, to a thickness of 0.4 mm. This raised the dielectric constantof the plastic foam material from 1.2 to 2.2. Therefore, due to thereduced thickness of the foam and the increased dielectric constant ofthe substrate material in the thinner portion 116, the total capacitancewas increased from 0.66 pF to 9.7 pF, which has a reactance of 17.8 ohmsat 915 MHz.

With reference now to FIG. 9, the RFID device 100 may include acompressed border or ridge edge 140 substantially fully surrounding thedevice 100. Part of the compressed ridge edge 140 serves as the thinnerportion 116 for capacitively coupling the second part 136 of theconductive tab 108 to the reflective structure 114. The remainder of thecompressed ridge edge 140 may serve a mechanical structural function,providing a rigid edge to the RFID device 100 to prevent flexing of theRFID device 100.

Another embodiment of the RFID device 100 is illustrated in FIG. 10. TheRFID device in FIG. 10 includes a resonator (a conductive tab) 150 witha capacitive ground 152 at one end. The wireless communication device106 is coupled to the resonator 150 at a suitable impedance point. Thewireless communication device 106 is also coupled to a capacitive ground154. The connection point between the wireless communication device 106and the resonator 150 may be selected to suitably match impedances ofthe wireless communication device 106 and the active part of theresonator 150.

The RFID devices 100 illustrated in FIGS. 8-10 may be suitable for useas labels, such as for placement on cartons containing any of a varietyof suitable materials. The RFID devices 100 may include other suitablelayers, for example an adhesive layer for mounting the RFID device 100on a carton, another type of container, or another object.

The RFID device 100 may be produced using suitable roll operations. FIG.11 shows a schematic diagram of a system 160 for making RFID devices,such as the RFID device 100. Beginning with a roll 162 of a substratematerial 164, a suitable printer 166 prints the conductive tabs 108 and110 (FIG. 8) and the reflective structure 114 (FIG. 8) on opposite sidesof the substrate material 164. It will be appreciated that the printer166 may actually include multiple printers, for example to print theconductive tabs in a separate operation from the printing of thereflective structure.

A placement station 168 may be used to place the wireless communicationdevices 106 (FIG. 8), such as straps. The wireless communication devices106 may be transferred to the substrate material 164 from a separate webof material 170. Alternatively, it will be appreciated that othermethods may be used to couple the wireless communication devices 106 tothe substrate material 164. For example, a suitable pick-and-placeoperation may be used to place the wireless communication devices 106.

Finally, the substrate material 164 is passed between a pair of rollers174 and 176. The rollers 174 and 176 may be suitably heated, and havesuitably-shaped surfaces, for example including suitable protrusionsand/or recesses, so as to compress a portion of the substrate material164, and to separate the RFID devices 100 one from another. In addition,a protective surface sheet 178 may be laminated onto the sheet material164, to provide a protective top surface for the RIFD devices 100. Itwill be appreciated that the compressing, laminating, and cuttingoperations may be performed in separate steps, if desired.

It will be appreciated that other suitable processes may be used infabricating the RFID devices 100. For example, suitable coatingtechniques, such as roll coating or spray coating, may be utilized forcoating one side of the devices with an adhesive, to facilitate adheringthe RFID devices to cartons or other containers.

The RFID device 100, with its monopole antenna structure 102, has theadvantage of a smaller size, when compared with similar devices havingdipole antenna structures. The length of the tag can be nearly halvedwith use of a monopole antenna, such as in the device 100, in comparisonto a dipole antennaed device having similar size of antenna elements(conductive tabs). By having RFID devices of a smaller size, it will beappreciated that such devices may be utilized in a wider variety ofapplications.

FIG. 12 shows an RFID device 180 having an expandable substrate 182,which can be maintained during manufacturing and processing operationswith a reduced thickness. The reduced thickness, which may be from about0.05 mm to 0.5 mm, may advantageously allow the RFID device 180 to passthrough standard printers, for example to print a bar code or otherinformation on a label 184 that is part of the RFID device 180. Afterperforming operations that take advantage of the reduced thicknesses ofthe substrate 182, the substrate 182 may be expanded, increasing itsthickness to that shown in FIG. 12.

The RFID device 180 has many of the components of other of the RFIDdevices described herein, including a wireless communication device 186and a pair of conductive tabs 188 and 190 on one side of the substrate182, and a reflective structure (conductive ground plane) 192 on theother side of the substrate 182.

Referring now in addition to FIGS. 13-15, details of the structure ofthe expandable substrate 182 are now given. The expandable substrate 182includes a top layer 202, a middle layer 204, and a bottom layer 206.The middle layer 204 is scored so as to be separated into segments 208,210, and 212, as a shear force is applied to the top layer 202 relativeto the bottom layer 206. The segments 208, 210, and 212 are in turnscored on fold lines, such as the fold lines 218 and 220 of the segment208. The scoring along the fold lines 218 allows parts 222, 224, and 226of the segment 208 to fold relative to one another as shear force isapplied between the top layer 202 and the bottom layer 206.

Each of the segments 208, 210, and 212 has three parts. The top layer202 has adhesive pads 232 selectively applied to adhere the bottom layer202 to the parts on one side of the segments 208, 210, and 212 (therightmost parts as shown in FIGS. 12-15). The bottom layer 206 hasadhesive pads 236 selectively applied to adhere the bottom layer 206 tothe parts on one side of the segments 208, 210, and 212 (the leftmostparts as shown in FIGS. 12-15). The middle parts of each of the segments208, 210, and 212 are not adhesively attached to either the top layer202 or the bottom layer 206, but are left free to flex relative to thesegment parts on either side.

With the expandable substrate 182 put together as shown in FIG. 14, thetop layer 202 and the bottom layer 206 being selectively adhered tosegment parts of the middle layer 204, other operations may be performedon the substrate 182 in its compressed state. For example, theconductive tabs 188 and 190 may be formed on the top layer 202, and thereflective structure 192 may be formed or placed on the bottom layer206. The wireless communication device 186 may be placed in contact withthe conductive tabs 188 and 190. Printing operations may be performed toprint on the label 184 of the RFID device 180. As noted above, thethickness of the compressed substrate 182 may allow the RFID device topass through a standard printer for printing the label or for performingother operations. In addition, the compressed substrate 182 may beeasier to use for performing other fabrication operations.

After fabrication operations that utilize the compressed substrate 182,the substrate 182 may be expanded, as illustrated in FIG. 15. When ashear force 240 is applied to the top layer 202 relative to the bottomlayer 206, the top layer 202 shifts position relative to the bottomlayer 206. The end parts of the segments 208, 210, and 212, some ofwhich are adhesively adhered to the top layer 202 and others of whichare adhered to the bottom layer 206, also move relative to one another.As the end parts of the segments 208, 210, and 212 shift relative to oneanother, the middle parts of the segments 208, 210, and 212 foldrelative to the end parts along the fold lines between the segmentparts. The middle parts of the segments 208, 210, and 212 thus deployand separate the top layer 202 and the bottom layer 206, expanding thesubstrate 182 and increasing the thickness of the expandable substrate182. The result is a corrugated structure. The expanded substrate 182has low dielectric loss in comparison with solid materials. With theincreased separation between the conductive tabs 188 and 190 due toexpansion of the substrate 182, the expanded substrate 182 is suitablefor use as a dielectric for a surface-independent RFID tag structure.

The shear force 240 between the top layer 202 and the bottom layer 206may be applied in any of a variety of suitable ways. For example, theshear force 240 may be applied by suitably configured rollers, with therollers having different rates of rotation or differences in grippingsurfaces. Alternatively, one of the layers 202 and 206 may include asuitable heat shrink layer that causes relative shear between the layers202 and 206 when the substrate 182 is heated.

The expandable substrate 182 may be fixed in expanded configuration byany of a variety of suitable ways, such as by pinning the ends of thelayers 202 and 206; sticking together suitable parts of the substrate182; filling gaps in the substrate 182 with a suitable material, such aspolyurethane foam; and suitably cutting and bending inward portions ofthe ends of the middle parts of the segments.

The layers 202, 204, and 206 may be layers made out of any of a varietyof suitable materials. The layers may be made of a suitable plasticmaterial. Alternatively, some or all of the layers may be made of apaper-based material, such as a suitable cardboard. Some of the layers202, 204, and 206 may be made of one material, and other of the layers202, 204, and 206 may be made of another material.

The RFID devices 180 may be suitable for use as a label, such as forplacement on cartons containing any of a variety of suitable materials.The RFID device 180 may include other suitable layers, for example anadhesive layer for mounting the RFID device 180 on a carton, anothertype of container, or another object.

It will be appreciated that the RFID device 180 may be used in suitableroll processes, such as the processes described above with regard to thesystem of FIG. 11. As stated above, the expandable substrate may be in acompressed state during some of the forming operations, for examplebeing expanded only after printing operations have been completed.

FIG. 16 illustrates an RFID device 260 that has a pair of generallyrectangular conductive tabs 262 and 264 that have a substantiallyconstant width along their length. More particularly, the conductivetabs 262 and 264 each may have a substantially constant width in adirection transverse to a longitudinal centerline axis of the tab. Theconductive tabs 262 and 264 form an antenna structure 270 that iscoupled to a wireless communication device 268 such as an RFID chip orstrap. The generally rectangular conductive tabs 262 have been found tobe effective when used in conjunction with conductive structures such asthe reflecting structures or ground planes described above.

It will be appreciated that the RFID device 260 is one of a wider classof devices having conductive tabs with substantially constant width,that may be effectively used with a reflective conductive structure.Such conductive tabs may have shapes other than the generallyrectangular shapes illustrated in FIG. 16.

Certain modifications and improvements will occur to those skilled inthe art upon a reading of the foregoing description. It should beunderstood that the present invention is not limited to any particulartype of wireless communication device, tabs, packaging, or slotarrangement. For the purposes of this application, couple, coupled, orcoupling is defined as either directly connecting or reactive coupling.Reactive coupling is defined as either capacitive or inductive coupling.One of ordinary skill in the art will recognize that there are differentmanners in which these elements can accomplish the present invention.The present invention is intended to cover what is claimed and anyequivalents. The specific embodiments used herein are to aid in theunderstanding of the present invention, and should not be used to limitthe scope of the invention in a manner narrower than the claims andtheir equivalents.

Although the invention has been shown and described with respect to acertain embodiment or embodiments, it is obvious that equivalentalterations and modifications will occur to others skilled in the artupon the reading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described elements (components, assemblies, devices, compositions,etc.), the terms (including a reference to a “means”) used to describesuch elements are intended to correspond, unless otherwise indicated, toany element which performs the specified function of the describedelement (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary embodiment or embodimentsof the invention. In addition, while a particular feature of theinvention may have been described above with respect to only one or moreof several illustrated embodiments, such feature may be combined withone or more other features of the other embodiments, as may be desiredand advantageous for any given or particular application.

1. An RFID device comprising: an expandable substrate; and an antennastructure on one face of the substrate; wherein expandable substrate maybe selectively expanded from a compressed state to an expanded state,increasing thickness of the expandable substrate by at least 50%.
 2. Thedevice of claim 1, wherein the antenna structure includes a pair ofconductive tabs on the one face of the substrate.
 3. The device of claim2, further comprising a conductive structure on a second face of thesubstrate, wherein the second face is on an opposite side of thesubstrate from the one face.
 4. The device of claim 1, furthercomprising a wireless communication device operatively coupled to theantenna structure.
 5. The device of claim 1, wherein the expandablesubstrate includes a top layer, a middle layer, and a bottom layer eachincluding at least one material piece separate from the other layers. 6.The device of claim 5, wherein at least one of the layers is made of apaper-based material.
 7. The device of claim 5, wherein one of the toplayer or the bottom layer is a heat shrink layer; and wherein heatingcauses relative shear between the top layer and the bottom layer.
 8. Thedevice of claim 5, wherein one of the top layer or the bottom layer is acompressible layer; and wherein pressure causes compressing of one ofthe compressible layer.
 9. The device of claim 1, wherein the expandablesubstrate is expanded by application of a mechanical force to thesubstrate.
 10. An RFID device comprising: an expandable substrate; andan antenna structure on one face of the substrate; wherein theexpandable substrate includes a top layer, a middle layer, and a bottomlayer; wherein the middle layer includes a plurality of segments;wherein each of the segments includes a first part, a second part, and athird part that is between the first part and the second part; whereinthe first part of each of the segments is coupled to the top layer;wherein the second part of each of the segments is coupled to the bottomlayer; and wherein the third part of each of the segments is rotatablerelative to the first part and the second part.
 11. The device of claim10, wherein each of the segments includes: a first fold line between thefirst part and the third part, and a second fold line between the secondpart and the third part.
 12. The device of claim 11, wherein the foldlines are scored portions of the segments.
 13. The device of claim 5,wherein the layers are plastic films.
 14. A method of forming the RFIDdevice that includes an expandable substrate; and an antenna structureon one face of the substrate,the method comprising: printing one or morelayers of the device atop the expandable substrate, with the substratein a compressed state; and after the printing, expanding the substratefrom the compressed state to an expanded state, thereby increasing thethickness of the substrate; wherein the expanding includes increasingthickness of the substrate by at least 50%.
 15. The method of claim 14,wherein the printing includes printing information on a label of theRFID device.
 16. The method of claim 14, wherein the expanding includesapplying a mechanical force to the substrate to increase the thicknessof the substrate.
 17. The method of claim 14, wherein having theexpandable substrate in the compressed state allows the substrate topass through a printer in which the printing is performed; and whereinthe expandable substrate in the expanded state is unable to pass throughthe printer.
 18. A method of forming an RFID device that includes anexpandable substrate; and an antenna structure on one face of thesubstrate, the method comprising: printing one or more layers of thedevice atop the expandable substrate; and expanding the substrate from acompressed state to an expanded state, thereby increasing the thicknessof the substrate; wherein the expanding includes imposing a shear stresson opposite faces of the substrate, to move one face of the substraterelative to another face of the substrate.
 19. The method of claim 18,wherein the substrate includes a top layer, a middle layer, and a bottomlayer; and wherein the imposing includes moving the top layer relativeto the bottom layer.
 20. An RFID device comprising: an expandablesubstrate having a first major surface and a second major surface,wherein the major surfaces are on opposite sides of the substrate; andan antenna structure on one of the major surfaces; wherein imposing arelative shear between the major surfaces selectively changes thicknessof the expandable substrate.
 21. The device of claim 20, wherein themajor surfaces are on separate respective top and bottom layers of thesubstrate that move relative to one another to selectively change thethickness of the expandable substrate.
 22. The device of claim 21,further comprising a middle layer between the top layer and the bottomlayer; wherein the middle layer includes a plurality of segments eachattached to both the top layer and the bottom layer.
 23. The device ofclaim 21, wherein the layers are plastic films.
 24. The device of claim21, wherein at least one of the layers is made of a paper-basedmaterial.
 25. An RFID device comprising: an expandable substrate havingfirst and second major surfaces on opposite sides thereof; an antenna onthe first surface; and a shield on one of the second surface, oppositethe antenna; wherein the expandable substrate may be selectivelyexpanded from a compressed state to an expanded state, increasingthickness of the expandable substrate by at least 50%.
 26. A method offorming an RFID device that includes an expandable substrate; and anantenna structure on one face of the substrate, the method comprising:printing one or more layers of the device atop the expandable substrate,with the substrate in a compressed state; and after the printing,expanding the substrate from the compressed state to an expanded state,thereby increasing the thickness of the substrate; wherein the expandingtransforms the substrate into a dielectric layer for surface-independentREID structure.